SEED-SCATTERING 275 



quietly in the country, to the energetic slinging of the 

 balsam (Impatiens noli-me-tangere). More extraordinary 

 is the almost animal violence of Hura crepitans, which 

 breaks into pieces, as if an explosive bomb, " with a report 

 like that of a pistol." In all cases what happens is, that 

 when the dying of the walls of the fruit reaches a certain 

 stage, there is a sudden release of certain tissue-tensions, 

 and hence the explosion. " In the case of the box (Buxus), 

 the smooth seeds are forcibly discharged by the contraction 

 of the pericarp, like a bean pressed between the fingers." 

 The pulling of the trigger is often due to the state of the 

 weather. 



Another simple method of dispersal is illustrated by a 

 few dry fruits which adhere readily to passing animals, 

 such as rabbits, and eventually fall off, it may be far 

 from the site of the parent plant. The little brown nutlets 

 of Jack-run-the-hedge or cleavers (Gallium aparine) are 

 covered with asperities which take a firm hold ; those of 

 the burdock have long crochet-needle-like hooks ; and 

 the awns of grasses are also very effective for adhesion. 

 The fruits of Medicago are common in the fleece of sheep, 

 but it is difficult to regard this as a profitable mode of 

 dispersal. 



The similarity of aquatic plants and animals in far- 

 separated freshwater pools is often very striking, and 

 part of the explanation is certainly, as Darwin pointed 

 out, that water-birds transport seeds and germs from 

 pool to pool on the mud attached to their feet. The same 

 is true of land-birds, which get clodlets fixed on their damp 

 feet. Darwin made a thorough study, after his wont, 

 of the fauna and flora of birds' feet, collecting the clodlets 

 and moistening them, to see what would come forth. 

 He proved up to the hilt the importance of this mode 

 of transport, and he was rewarded on one occasion by 



